Transmitting signals through imperfectly-conducting media is a notoriously difficult problem. A major reason for this difficulty is that imperfect conductors severely attenuate radio waves traveling through them. This is because, as discussed in Jordan and Balman, “Electromagnetic Waves and Radiating Systems”, Prentice-Hall, 1968, Chapter 5, imperfect conductivity (partial conductivity) causes attenuation of the electric field component of the oscillating electric/magnetic energy wave, such as a propagating radio signal. This attenuation renders radio communication under water nearly impossible. Even where possible, such communication is generally impractical. Consequently, its use is limited to only a few applications. For example, using very low frequencies and very high power levels, radio waves can be transmitted into deep water whereby communications with submarines generally require high power transmitters that transmit signals containing frequencies below approximately 10 KHz.
Largely because of the difficulties associated with transmitting electromagnetic waves through imperfectly conducting media, most systems that try to transmit signals through such media use acoustic energy, rather than electromagnetic energy. Examples of such systems are found in the “DiveLink” ultrasonic system sold by Divelink, Inc. and the “Buddy Phone” sold by Ocean Technology, Inc. However, acoustic systems also suffer from a number of drawbacks. One drawback is that, like electromagnetic waves, acoustic waves suffer significant attenuation in water or earth. See Urick, R., J., “Principles of Underwater Sound”, 3d Edition, McGraw-Hill Book Company, New York, 1983.
Another drawback is that natural or man-made noise can interfere with acoustic systems. For example, acoustic noise from surf or storms or engine noise from nearby boats can dramatically affect the performance of underwater acoustic communication
Another problem with acoustic signaling arises from reflections that can occur when properties of the medium through which an acoustic wave propagates vary. An exemplary change in a property of a medium is a thermocline in water. Although useful in some applications such as SONAR, reflection of acoustic waves in a communication system is generally detrimental to the communications. For example, due to the relatively slow speed of sound propagation in water, reflection of acoustic waves can lead to severe multi-path interference, which causes degradation in intelligibility and loss of communication bandwidth. Moreover, in some cases, the reflection is so severe that it causes complete loss of signal results due to reflection of the acoustic signal wave away from the desired transmission path.
There has been little research exploring the use of the imperfect conductivity of the medium as a beneficial feature, rather than a detriment, to communication systems. In U.S. Pat. No. 4,207,568 to MacLeod, a communication link is described that uses the bulk conductivity of water for one side of a transmission circuit, and a water-filled, flexible insulating tube as the other side of the circuit. Although this approach avoids the problems of non-flexible conductive wires, it requires the tube to make a physical connection between the ends of the communication link. Consequently, it is limited in its application.
There also has been work done in transmitting data using the body to form a personal-area network. This is essentially using the body as an antenna. For example, in U.S. Pat. No. 6,754,472 B1 to Williams et al, a network for a variety of small devices worn by a person is described. However, these networks do not provide for stimulation output or for other outputs to affect the body. Also, the frequencies described in the Williams et al patent are within a range that would cause shock and unwanted muscular stimulation at the levels required of the present invention.
The BION system (See Loeb, et. al., Journal of Medical Engineering and Physics, Vol. 23, pages 9-18) is a wireless system to provide stimulation outputs from a small, implanted system. However, the BION system does not use an electrical transmission method, instead the BION system uses a magnetic field that is picked up by a receiving coil in the implanted device. This requires a large coil with very high current to be placed on the body in close proximity to the receiving device. Providing this coil can be very cumbersome.